Previously, in order to prevent adhesion of barnacle, sea lettuce and other marine organism to ships and various submarine structures, triorganotin polymer compounds have been generally used, but use thereof has been considerably limited due to marine pollution in recent years. In order to solve this problem, various underwater antifouling coating compositions using a metal pyrithione compound and cuprous oxide as an active ingredient have been proposed (U.S. Pat. No. 5,057,153 and JP-A No. 53-27630).
In addition, as a vehicle for an underwater antifouling painting agent, there is known a hydrolyzable resin composition containing at least one kind of a group, for example, represented by the following formula (III):
(wherein R represents hydrogen or an alkyl group of a carbon number of 1 to 4, M3 represents Si, Cu or Zn, and L represents an alkyl group when M3 is Si, or an organic acid residue when M3 is Cu or Zn, and n is the number obtained by subtracting 1 from a valence of M3)    in a molecule constituting a resin such as an acryl resin, as a so-called hydrolyzable resin which is gradually hydrolyzed in water and renews a surface of the coating film thereof is (JP-A No. 10-101969, JP-A No. 10-158547, JP-A No. 10-168350, JP-A No. 2000-265107, JP-A No. 2000-273365, JP-A No. 2000-273366, JP-A No. 2000-273384, JP-A No. 2000-273385, JP-A No. 63-128008, JP-A No. 11-302572 etc.).
However, it is know that, when an antifouling painting agent is prepared using a metal pyrithione compound and cuprous oxide as an active ingredient, aggregation or thickening is frequently caused during storage and, in the remarkable case, gelation is caused.
For example, in the case of a coating composition containing zinc pyrithione (ZPT) and cuprous oxide as an active ingredient, and a copper acryl resin as a vehicle, a denatured substance is produced as a byproduct in a composition by a metal exchange reaction from ZPT to copper pyrithione CPT, and causes aggregation or thickening, consequently gelation.
In addition, in the case of a coating composition containing CPT and cuprous oxide, it is said that aggregation or thickening and, occasionally, gelation of the coating composition is caused depending upon the quality of cuprous oxide, particularly, an amount of a metal copper included in the composition.
An underwater antifouling painting agent having improved storage stability that contains metal pyrithione and cuprous oxide was studied, and several patents have been already proposed (U.S. Pat. Nos. 5,098,473, 5,112,397, 5,137,569). However, these painting agents also have insufficient storage stability and, moreover, cause for this storage instability has not been sufficiently clarified now yet. Therefore, a problem as to how a coating composition can be stabilized regardless of quality of metal pyrithione and cuprous oxide has not been solved.